There is no such form.
When you file your tax return, you self-certify that you are drawing from contributions, based on your own accurate financial records. You don't submit any proof with your tax return. If you are audited, you must be able to prove, using your own reliable records, that you were only withdrawing contributions. Records that will help prove this include copies of form 5498 (showing contributions made each year) and any prior 1099-R forms showing prior withdrawals.
Even if you had a CPA who would write such a letter for you, that would not be proof if you were audited, you would still need to show the IRS your records. (Although, if you did have a CPA, presumably you pay them to help you keep those records to show the IRS.)
